It is widely accepted that silicon nitride has excellent intrinsic properties such as strength, hardness, oxidation, creep and thermal shock resistance. However, because of the highly covalent nature of the chemical bonding in this material and corresponding low diffusion rates, traditional ceramic fabrication techniques do not lead to dense bodies. In order to achieve low porosity materials, densification aids are necessary which compromise the properties of fabricated specimens especially at elevated temperatures.
A development was the discovery that aluminium could be incorporated into the lattice structure of silicon nitride. The material so formed was believed to be a single phase solid solution of alumina in silicon nitride. Early workers found this material was easily fabricated using well known ceramic techniques However, it was subsequently reported that this material is multi-phased and that the original formula was incorrect. Thus, the solid solution was not as previously proposed but existed between silicon nitride and aluminium oxynitride Al.sub.3 ON.sub.3. The properties of the early materials were inferior to those of silicon nitride as a result of a residual grain boundary glass which also explained the greater ease of densification. The correct formula for aluminium substituted silicon nitride (designated .beta.-sialon) is EQU Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z where z.ltoreq.4.2 at 1750.degree. C.
In order to sinter these materials to high density, sintering aids are necessary.
Another non-oxide ceramic with excellent intrinsic properties is silicon oxynitride. This material has been used as a bonding phase for silicon carbide refractories, and it has been reported that limited replacement of silicon and nitrogen by aluminium and oxygen can also occur in silicon oxynitride (O-sialon).
It has long been appreciated that rice hulls can be utilized as a raw material in the manufacture if silicon nitride and silicon carbide powders both in whisker and particulate forms. At present rice hulls pose a considerable waste disposal problem. With Australian rice production levels as high as 830 000 tonnes (1982), up to 160 000 tonnes of rice hulls are being produced annually. In the Australian context, if all the rice hull waste produced by the milling of rice was used to make silicon nitride, up to 20 000 tonnes could be produced per annum based on recent figures for the production of rice.
Rice hulls contain approximately 20 weight percent silica with the bulk being organic material. Pyrolysis of the rice hulls leads to a 60 percent weight loss. The resultant material consists of approximately 55 weight percent silica and 45 weight percent of carbon Both are in a finely divided state and intimately mixed. Upon ashing, a material containing in excess of 95 weight percent silica can be obtained Elements such as iron, aluminium, sodium, potassium, calcium and magnesium may also be present.
The formation of silicon nitride and silicon carbide from rice hulls involves a number of steps. The rice hulls are pyrolysed to decompose the organic component to carbon. This material is then heated at temperatures between 1000.degree. to 2000.degree. C. to form the silicon carbide or nitride. Early work on the formation of silicon carbide and nitride from rice hulls is the subject of United States parent specifications Nos. 3754076 and 3855395.
A problem with the conventional method for the manufacture of dense ceramic materials from the products of the carbothermal reduction of silica is that, after the process, unreacted silica can be present and must be separated from the product. This usually involves a leaching process. Additions of catalyst can be made to increase the reaction rate, and a common additive is iron. However, the presence of such additives has been shown to be detrimental, leading to impaired mechanical properties and decreased oxidation resistance as well as promoting the decomposition of silicon nitride during sintering.
The production of materials based on .beta.-sialons, utilizing the reaction products of a carbothermal reduction has been proposed in U.S. patent specification No. 3960581. The proposed reactants for the fabrication of these ceramic components were 15 to 70 weight percent alumina and 85 to 30 weight percent silicon nitride. However, it is believed that this reaction produces ceramic materials with properties inferior to those of silicon nitride and corresponding .beta.-sialons.
It is an object of the present invention to produce a ceramic product which may be readily densified and which has desirable intrinsic properties and there is accordingly provided a method of forming a ceramic product by converting silica to silicon nitride in a carbothermal reduction process in the presence of nitrogen, fabricating the silicon nitride into the desired shape of the ceramic product and sintering the fabricated silicon nitride, characterised in that the carbothermal reduction process is not completed so as to produce a mixture comprising residual silica and silicon nitride which is fabricated into the desired shape and is heated in a nitrogen atmosphere at a temperature of from 1400.degree. C. to 2000.degree. C. to produce a ceramic product comprised of silicon nitride and silicon oxynitride A low or controlled oxygen partial pressure may be present in the heating atmosphere. The oxygen may be derived from the reaction or entrained in the heating atmosphere.
Further according to the present invention there is provided a ceramic product when formed by the method s described in the immediately preceding paragraph. The product may comprise from 1-99 weight percent silicon oxynitride and from 1 to 95 weight percent silicon nitride. Generally there will also be a gassy phase present in the ceramic material, but this may have a thickness of only a few nanometers at the grain boundaries and represent as little as 0.01 weight percent of the ceramic material. The glassy phase at this level may be composed of impurities in the initial materials, such as calcium, magnesium and aluminium and may also contain silicon, oxygen and nitrogen so that it can be present as silicates and oxynitrides.
We have found that the fabricated mixtures can be readily densified and provide advantageous intrinsic properties for use in fields similar to silicon nitride and silicon oxynitride.
Sintering aids can be used to assist densification and these can be added as the metal, oxide, nitrate, carbonate or nitride of the following elements: lithium, beryllium, magnesium, calcium, scandium, yttrrum, cerium, titanium, zirconium, hafnium and members of the rare earth group. Sintering can be performed with or without external pressure. The sintering aids tend to go into the glassy phase which may comprise up to about 20 weight percent of the ceramic product.
The ceramic product may also contain up to 40 weight percent silicon carbide, particularly where this is a reaction product of the incomplete carbothermal reduction process. Silicon carbide is formed in the carbothermal reduction process if there is an insufficient nitrogen partial pressure to convert the silica only to silicon nitride, or if the process is carried out in a temperature range of, for example, from 1500.degree. to 1600.degree. C.
At least part of the silicon oxynitride and silicon nitride in the ceramic product may be present in the form of one or more aluminium substituted derivatives such as .beta.-sialon and O-sialon. Thus the main crystalline phases may obey the general formulae: EQU Si.sub.2-x Al.sub.x O.sub.1+x N.sub.2-x where x.ltoreq.0.2 and EQU Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z where z.ltoreq.1.
Thus, in addition to the silica and silicon nitride used in the method of the present invention, the fabricated mixture may conveniently comprise one or more of the following finely divided particulate materials: silicon carbide, silicon, alumina, aluminium nitride, aluminium hydroxide and aluminium silicates. Preferably the fabricated mixture comprises 0.3 to 36 weight percent silica, 0.5 to 15 weight percent alumina, up to 15 weight percent aluminium nitride, up to 40 weight percent silicon carbide and from 34 to 97 weight percent silicon nitride. Most preferably, the silica content is from 5 to 30 weight percent, the alumina content is from 3 to 10 weight percent and the silicon nitride content is from 60 to 90 weight percent.
The ceramic product may further comprise whisker or fibre reinforcement and this is conveniently based on the elements silicon, aluminium, oxygen, nitrogen and carbon.
A very substantial advantage of the present invention is that all or some of the finely divided particulate materials in the fabricated mixture may be derived from the incomplete carbothermal reduction of silica. The silica may be derived from clays (also generally containing alumina) and other well known sources, but most conveniently is derived from the product of the pyrolysis of rice hulls which generally contains about 0.25 weight percent aluminium in oxide form. Where the precursor powders are derived from treated rice hulls the rice hulls may be pyrolysed at from 250.degree. to 1000.degree. C. to decompose the organic components and then heated in a nitrogen atmosphere at between 1000.degree. to 2000.degree. C. for a sufficiently short period that not all of the silica present undergoes carbothermal reduction, and subsequently given a low temperature heat treatment to remove residual carbon and to yield a mixture of silica and silicon nitride with or without silicon carbide. The purity of the pyrolysis product can be increased by the use of relatively simple treatments such as water or acid extractions. Investigations have shown that water and acid extractions performed on rice hulls have yielded silica with greater than 99 and 99.5 percent purity respectively. This step is most effectively carried out on the rice hulls before the pyrolysis operation.
The low temperature heat treatment may be performed at a temperature of 500.degree. C. to 1000.degree. C., for example at 700.degree. C., in an oxygen containing atmosphere to remove the excess carbon.
It may be advantageous to include one or more other gases, such as hydrogen, in the nitrogen carbothermal reduction atmosphere. Preferably the carbothermal reduction is performed over a time period of from 6 hours to 30 hours in a preferred temperature range of 1300.degree. C. to 1450.degree. C.
The complete carbothermal reduction of silica is as follows: ##STR1##
However the incomplete carbothermal reduction step will produce as end products both silica and silicon nitride as well as possibly silicon carbide. Alumina can be added into the carbothermal reduction step, or thereafter if none or insufficient is already present. The main precursor powders formed by the controlled reaction of silica and carbon can be produced with crystalline morphologies with either high or low aspect ratios or a combination of both types. Particle size of the powders, which will usually be equiaxed, is preferably less than 5 .mu.m and most preferably less than 0.5 .mu.m. When whiskers formed in the carbothermal reduction of silica are used in the fabricated mixture they preferably have diameters less than 2 .mu.m , most preferably less than 1 .mu.m while advantageously being greater than 5 .mu.m long and preferably longer than 10 .mu.m. Any aluminium containing species present preferably has a particle size of less than 10 .mu.m. The powder may be milled to break up any agglomerates and the aluminium species may be added at this stage.
The fabrication and densification step may be as follows: ##STR2## where approximately EQU 0.01 a.ltoreq.25 EQU 0.1 b.ltoreq.33 EQU 4a+3b=100 EQU x.ltoreq.0.2 EQU z.ltoreq.1
The overall composition of the fired materials, except for any silicon carbide present, may accordingly be bounded approximately by the following compositions:
(1) Si.sub.2 N.sub.2 O PA1 (2) Si.sub.1.8 Al.sub.0.2 O.sub.1.2 N.sub.1.8, PA1 (3) Si.sub.5 Al.sub.1 O.sub.1 N.sub.7 and PA1 (4) Si.sub.3 N.sub.4.
The ceramic product preferably has a fine grained microstructure with grains less than 5.mu. in size, with or without high aspect ratio grains with aspect ratios 20 greater than 5.